138 
MA OTE 
| DECEMBER 6, 1906 
RECENT PROGRESS IN MAGNETO-OPTICS.* 
is my intention this evening to give you a general 
Tr 
I review of the experimental researches which have 
occupied me during the last few years. They all refer to 
the relation between magnetism and light, a relation the 
first and fundamental example of which was discovered in 
this very institution by Faraday in 1845. 
Surely every physicist should feel inspired by the idea 
of having the privilege to address an audience in the same 
lecture room, where so often some of nature’s deeper 
mysteries were revealed; and I feel the uplifting force of 
this inspiration all the stronger, as my own work for 
many years has been so closely connected with one of 
Faraday’s discoveries. Faraday discovered that the plane 
in which the vibrations of light take place rotates when- 
ever a ray of light is propagated parallel to the magnetic 
lines of force through some substances, such as Faraday’s 
own heavy glass; this fact we now indicate by the term 
the magnetic rotation of the plane of polarisation. The 
discovery of this fact opened the chapter of magneto- 
optics. 
Faraday’s mind again and again returned to the relation 
between magnetism and light, and incessantly he sought 
for closer and more intimate connections; in one experi- 
ment in March, 1862 (which is said to have been his last), 
he tried to observe a change in the spectrum of a flame 
when acted on by a magnet. The entry in Faraday’s note- 
book, preserved in this institution with pious care, con- 
cludes with the words, ‘‘ not the slightest effect on the 
polarised or unpolarised ray was observed.*’ As we now 
know, the means of Faraday’s time were not powerful 
enough to observe the effect sought for. Various physicists 
since Faraday have sought in the same direction; some 
have recorded their negative results, others have not, for 
most physicists have an almost invincible dislike for the 
publication of negative results, though a collection of such 
unsuccessful attempts, if precisely stated, would be most 
interesting, and should afterwards prove very valuable. 
Magnetisation of the Spectral Lines. 
In my own case, the thought to submit a source of light 
to the influence of magnetism occurred to me during a 
quantitative investigation of the effect discovered by Kerr 
concerning the light reflected by magnetised mirrors. I 
was working at the time in Leyden, in Prof. Onnes’s 
laboratory. The account of Faraday’s negative experiment 
encouraged me in my endeavours, and also an argument 
in 1856 by Lord Kelvin, referred to by Maxwell as the 
“exceedingly important remark of Sir W. Thomson.’’ If 
it might be accepted that the forces operating during the 
propagation of light in magnetised substances exist also 
whenever the source of light is in the magnetic field, we 
can expect some direct effect of magnetism on radiation. 
My own successful experiments date from 1896 to 1897, 
whereas three years earlier I also had recorded a negative 
result, not having then used adequate means. 
As you know, a sodium flame chiefly emits two kinds 
of yellow light, and accordingly its spectrum, when 
analysed with one of Rowland’s large concave gratings, 
shows two yellow lines. With a grating of medium size 
these lines have a distance of one millimetre; they are 
rather narrow as shown in the slide. In August, 18096, 
I found that when a sodium flame is placed between the 
poles of an electromagnet, and is looked at with a spectro- 
scope in a direction at right angles to the lines of force, 
the yellow lines in its spectrum) become somewhat. wider 
when the magnetic field is put on.” This fact can be ex- 
pressed in a different way by saying that, besides the 
original vibrations, a flame in « magnetic field emits other 
vibrations, of which some have a somewhat greater, and 
some a somewhat smaller, frequency than the original 
vibrations. 
This observation of a small change in a spectral line 
was the origin of my subsequent work. I realised that 
this change, however small, was worth a closer examin- 
ation. Indeed, it seemed clear at once that here we had 
1 Discourse delivered at the Royal Institution on Friday, March 30, by 
Prof. P Zeeman. rs : 
2 Zeeman, Verslagen Kon. Akademie v. Wetenschappen, Amsterdam, 
October and November, 1896. P77. Mag., March, 1897. 
NO. 1936, VOL. 75] 
a means of studying the internal vibrations of a molecule 
by modifying in a simple way the conditions under which 
they are going on. Of course, the result was verified in 
all directions. As there is now, I think, no doubt as to 
the reality of the observed changes, I shall only refer very 
briefly to this stage of the work. In the first place, the 
widening of the lines was observed in the direction of the 
lines of force also. Then the fact was established that to 
the observed direct effect there corresponds an inverse one. 
When white light traverses the incandescent sodium vapour 
we observe the absorption lines; these also are widened 
when the vapour is subjected to magnetic forces. Secondary 
influences were discarded by suitable modifications of the 
experiments. In one case no change was observed. The 
spectra of fluted bands, such as those of iodine, carbon, 
or nitrogen, did not show any effect, nor could Becquerel 
and Deslandres using increased power discover it. 
Before I could answer the different questions which 
presented themselves, I had the advantage that the beau- 
tiful theory of the electromagnetic and optical phenomena, 
developed by my friend Prof. Lorentz, gave its quickening 
influence to my experimental work. 
In this theory it is supposed that the material world is 
built up of three things: ponderable matter, ether, and 
electrons. I think it is rather superfluous to remind you 
here, in the land of Maxwell, Kelvin, Crookes, J. J. 
Thomson, Schuster, Larmor, Heaviside, and Johnstone 
(EF: 
, Fic. 1. 
Stoney, that electrons or corpuscles are exceedingly small, 
electrically charged particles, which are supposed to be 
present in all material bodies. 
These electrons can perform oscillations under the in- 
fluence of the forces which attract them to their position 
of equilibrium. Because they are electrified they have 
sufficient hold on the ether to excite in it the electro- 
magnetic vibrations which, according to Maxwell’s theory, 
constitute light. The oscillatory periods of the electrons 
determine the position of the lines in the spectrum, and 
with every change in the period of oscillation we observe 
a displacement of the corresponding line. 
In Lorentz’s theory the explanation of the effect of a 
magnetic field is as simple as it is beautiful. 
The forces operating on the vibrating electron in a mag- 
netic field are fairly well known. These forces are the 
same which curve the path of the kathode rays in a 
vacuum tube which is acted on by a magnet. All motions 
of the electrons in the molecules of a flame may be sup- 
posed to be made up of three particular motions, chosen 
in such a manner that the action of the magnetic field 
on each of them can be easily foreseen. The light of the 
flame is exactly the same as it would be if the flame con- 
tained three groups of electrons vibrating in these simple 
ways. In this model the electrons are represented by red 
balls; the black arrow indicates the direction of the mag- 
netic force (Fig. 1). 
